An acoustic well log is a continuous piecewise measurement of the vertical velocity within the sediments by a borehole. Formation velocities are measured directly by a tool lowered down the borehold on a precisely calibrated cable so the precision of measurement is fairly constant at any depth. Studies made by a committee of the Canadian Petroleum Association and others demonstrates an acoustic log to have a resolving power of about one foot of formation thickness or less, and to detect velocity change of 1%.
Each of the units in a sedimentary column falls within a relatively narrow, but not necessarily unique, range of velocities. Thus, measurements of the velocities can provide some indication of the nature of sediments penetrated.
Changes in the characteristic velocities are also very important. Sudden changes in velocity mark the boundaries of stratigraphic units, enabling geologists to correlate stratigraphy from location to location, and thereby to construct maps of the several subsurface units.
A decrease in velocity within a unit can be diagnostic of increasing porosity, a favorable condition for a petroleum reservoir formation, and a further reduction may indicate the presence of hydrocarbon, in particular natural gas, within the reservoir formation.
For these, and other reasons, the acoustic well log is a valuable tool for the petroleum geologist, and forms one of several measurements of natural physical phenomena associated with the earth's crust which are usually made in any hole drilled for petroleum.
Seismic reflection surveying is another petroleum exploration tool in common use. The typical procedure is to detonate a small explosive charge buried a few feet below the earth's surface. Sound waves from the explosive radiate through the earth's crust and are reflected much like an echo from formation boundaries back to the surface, where their arrival is detected by extremely sensitive seismometers. Accurate measurement of the time between the initial pulse and the arrival of the reflected energy provides a means of estimating the depth to the reflector.
In practice, the information detected by the seismometer is recorded continuously for several seconds following the detonation of the explosive. In this manner, echoes from several beds are obtained in a continuous sequence. Repeating the process at several locations permits continuous, simultaneous mapping of several of the major geological units that make up the earth's crust.
The seismic measurements are in units of time, commonly milliseconds. The reflection times of seismic events are normally measured to one millisecond precision. Since the reflection is a two-way time, down and back, it should be possible to measure depth changes of 0.5 millisecond. At an average velocity of precisely 10,000 feet per second, this should provide the ability to measure differences in depth as small as five feet.
Determination of the absolute depth to a given formation can be made only if the velocity of seismic waves through the earth at that point is known.
Very precise velocity information can be obtained from the acoustic log, but this requires drilling a deep borehole. Much less precise velocity measurements have been made from the surface by recording the seismic reflection from a common subsurface point at two separate locations. The difference in travel times between two receivers separated by a known distance can be used to solve for velocity by simple trigonemetry. However, the results are only approximate at best, and may be distorted by anistrophy within the earth and aberration in the travel paths followed by the energy to the two observation stations. Subsurface velocities obtained from the surface by this method have rarely been more accurate than about 2%, or 200 feet in 10,000 feet, and are very insensitive to sudden change.
Therefore, although the seismic method may be able to measure relative structural elevation changes with considerable precision, it has not been very precise for absolute depth measurement.
Furthermore, the resolution of individual subsurface geologic units by the seismogram is rather limited. The ground acts as a frequency filter of seismic waves. Often the upper limit of useful returning seismic frequencies on conventional seismograms is limited to about 50 Hertz. Seismic waves may contain weak components up to approximately 100 Hertz, but these are normally discarded. A signal having a frequency of 50 Hertz has a period of 0.020 second. At a velocity of 10,000 feet per second, the wavelength of that signal becomes 200 feet. Thus, at best, a seismic wave must travel down and back through a bed of at least 100 feet thickness in order to distinguish it from an adjacent bed. The results of normal production surveys probably have reliable absolute resolving powers of only about half of the ideal, say 200 feet. The presence of much thinner beds, to about 1/8 wavelength or less, may be detected but thier thickness cannot be precisely measured.
Thus, although the acoustic log and the seismic reflection trace both measure acoustic properties of the sedimentary section, the seismic information cannot provide the detailed resolution of thin beds nor of velocity changes possible to measure with the acoustic log. The seismic method does have the great advantage that measurements can be made from the earth's surface without the need to drill a deep borehole. However, due to its limited powers of resolution of velocity and bed thickness, its application has been limited almost exclusively to mapping the shape of subsurface structure by measuring reflection travel times.
The seismic signal is usually distorted in its travel through the earth, and often buried in noise. The quality and resolution of production seismic field data may be considerably degraded by this distortion and noise.
A common procedure which has been applied to the signal to improve its quality is bandpass frequency filtering. Of the serveral types of noise which may be present in a seismic signal, the two most common are broadband random noise and low frequency near-surface noise waves produced as a by-product of the source energy. Of the two types, the low frequency energy say below 15 Hertz often is the strongest and may completely overwhelm the signal.
At the other end of the frequency scale, ground effects normally cause a rapid increase in attentuation of seismic reflection frequencies above 40 Hertz. Random noise may be fairly uniform in amplitude across the frequency spectrum but very commonly increases in strength at higher frequencies. In either case, although the seismic signal amplitude may be strong at frequencies below 40 Hertz, above that point it falls rather quickly, so that at some point, say 60 Hz., it will fall below the amplitude level of the noise. Quite frequently, the presence of the seismic signal may be made more obvious by eliminating all frequencies above that point where the noise level is greater than the signal.
As a result, conventional seismic data is commonly limited to a bandwidth of about 15 to 55 Hz., or just under two octaves bandwidth.
For use in structural mapping, frequency band limiting of a seismic signal to reject noise is often a very effective procedure, leaving the signal in the remaining portion of the spectrum sufficiently free of noise to enable clear definition where none could be detected before.
From a theoretical standpoint, however, application of a bandpass filter to a seismic signal reduces its information carrying capacity. The information capacity of any signal is a product of its time duration, frequency bandwidth, and amplitude. During the past decade, many specialized field data acquisition instruments and techniques and procedures have been developed to improve seismic data quality by eliminating noise and correcting signal distortion.
One highly successful technique, termed stacking, patented by Mayne (U.S. Pat. No. 2,732,906), records in the field several redundent observations of the same subsurface point. After suitable adjustments and corrections, the several observations are summed and averaged, to cancel noise and enhance the signal. This procedure is often effective in extending the range of useful frequencies above the normal limit of 50 Hertz found on conventional seismic data.
One of the principal digital procedures applied to enhance seismic data is termed deconvolution. Briefly, this procedure uses mathematical least-squares techniques to determine, under certain assumptions and within certain limiting constraints, the probable distortion and attenuation that has occurred to the frequency spectrum of a seismic signal in passage through the earth. From this determination a correction is calculated and applied to the signal to compensate for the distortion, and attenuation. When this process is applied to stacked data useful high frequency information can be developed, and the resolution of the seismic signal is often improved. The relative effects of the bandwidth reduction upon a signal already having limited resolving power is often given little attention. At present, of the foregoing components of the signal, only the component of time is used to any great extent.
For use in structural mapping, frequency band limiting of a seismic signal to reject noise is often a very effective procedure, leaving the signal in the remaining portion of the spectrum sufficiently free of noise to enable clear definition where none could be detected before.
The third component, amplitude, has also generally been ignored. In most processing it is merely set to a constant or nearly so.
In U.S. Pat. No. 3,671,930, Mateker, Jr., there is disclosed a method of exploration using seismic techniques wherein an amplitude-time recording of reflections from strata is made. This recording function is divided by a second recording function converted into a reflectivity function, and the logarithm of the result is derived to approximate a function representative of the pattern of velocities through the strata in the geologic section. This technique is based on the assumption that the amplitude of the acoustic signal passing through the strata decreases exponentially with time (depth). While this assumption leads to a good approximation and provides better information than previous techniques, the assumption is only completely valid in sections where the reflection coefficient of interfaces between strata are equal to each other or follow a definite pattern, conditions which are seldom fully met. Also, the procedure measures ratios only, and many combinations of sediments produce similar ratios. In another recent development, sudden extreme changes in seismic reflection amplitude are related to the pressence of gas in the formation.